1. Field of the Invention
The present invention relates to a forced-air-cooled engine having a cylinder block which is composed in one piece with a crankcase part and a cylinder part, specifically to a vibration control supporting method and a vibration control supporting construction of a cooling fan cover which constitutes a casing for guiding cooling air to the cylinder block.
2. Description of the Related Art
The forced-air-cooled engine to which the present invention is applied is one having a crankshaft supported horizontally in the crankcase part of the cylinder block by a pair of bearings and having a cooling fan attached to an end side (opposite to an output shaft side) of the crankshaft. The cooling fan rotates with the crankshaft and produce cooling air flow which hits the cooling fins formed around the outer surface of the cylinder part protruding from the crankcase part radial direction of the crankshaft to cool the cylinder.
As the fan cover constituting the casing of the cooling fan is fixed to the cylinder block, the fan cover vibrates violently resonating to the vibration of the cylinder block, as a result, not only noise is generated but also loosening and dropping-out of adjacent parts are induced that often lead to trouble. Thus, forced-air-cooled engines have been faced with a problem of reduced reliability.
A variety of constructions to fix the fan casing to the cylinder block have been devised in the field of forced-air-cooling engines for preventing resonance with engine vibration. There are disclosed vibration isolation supporting between the fan cover and cylinder block in Japanese Patent No. 2691461 (hereafter referred to as patent literature 1) and Japanese Published Utility Model Application No. 61-33224 (hereafter referred to as patent literature 2).
Vibration control supporting are adopted in these disclosures is to attach the fan cover to the cylinder block by the medium of rubber grommets fit in a plurality of holes provided along the peripheral part of the fan cover and bolts passing through holes of the grommets so that the fan cover is supported elastically.
With the art, natural frequency of the fan cover can be lowered than frequencies of engine vibration occurring at normal rotation speed range of the engine by properly determining material and size of the rubber grommet and adjusting tightening force of the bolts. However, in this method, the fan cover is supported at a plurality of fixed positions elastically via the rubber grommets and bolts, so when unexpected force is transmitted to the fan cover from the cylinder block side or crankshaft side, the fan cover is moved from side to side and up and down, that is, the cooling fan cover is not stable against unexpected force.
Further, as a plurality of short cylindrical holes for fitting in rubber grommets must be formed by drawing along the peripheral part of the fan cover for receiving the rubber grommets, manufacturing process of the fan cover becomes complicated.
There is disclosed in Japanese Patent No. 3686202 (hereafter referred to as patent literature 3) another art of attaching a cooling air guide casing, which is not constructed such that the cooling air guide casing is fixed to the cylinder block at a plurality of fixing points positioned at certain spacing by means of bolts via rubber grommets to support the fan cover elastically, but in which two damper plates for insulating vibration are placed between the cylinder block and the cooling air guide casing along the peripheral border of the cooling fan casing for preventing amplification of engine vibration and noise thanks to vibration damping capacity of the damper plate. A concretive vibration control method is disclosed in said patent literature, in which the cooling air guide casing (cooling fan cover) is fixed to the cylinder block with two damper plates which are formed to serve as cooling air guide plates being disposed between the air guide casing and cylinder block in a state the damper plates are pressed against the cylinder block by the tightening force of a plurality of bolts.
With this art, when elasticity of the damper member and tightening force of the bolts are properly determined, transmission of vibration of the cylinder block to the cooling air guide casing is suppressed by virtue of vibration damping capacity of the damper member, and noise generation due to vibration, particularly resonance of the cooling air guide casing can be prevented.
However, it is not easy to machine with accuracy the concaved parts for receiving the damper plates to be of depth so that the damper plates are attached between the air guide sassing and the end face of the cylinder block with proper interference, or shrink of the damper plate made of synthetic resin when the air guide casing is fixed to the cylinder block, taking into consideration the elasticity of the synthetic resin material.